In a color picture tube having a shadow mask, the mask is arranged in direct proximity to the interior surface of the screen. Because the luminescent segments are produced on the interior surface of the screen, the geometry of the shadow mask is required to conform with the pattern of the luminescent segments when the color picture tube is in operation. Maximum impact accuracy of the electron on the luminescent segments is achieved when the aperture geometry of the shadow mask conforms with the distribution of the luminescent segments on the interior surface of the screen at the operating temperature. However, since only a small portion of the emitted electrons pass through holes in the mask and strike the luminescent segments and the majority of the electrons strike the mask directly, the mask is heated up to 80.degree. C. as a result, giving rise to a change in mask geometry which results in doming of the mask (doming effect).
After doming, the aperture geometry of the shadow mask no longer conforms with the pattern of the luminescent segments, giving rise to imprecise electron impact. The color rendering quality of the screen is disturbed.
With high contrast pictures, different areas of the mask will be heated up to different levels, thus giving rise to partial doming of the mask (local doming) which also results in aberrations when the doming exceeds a tolerance.
A variety of attempts have been made to limit or prevent such disadvantageous thermal behavior of the shadow mask. Thus, various measures have been suggested to limit excessive heating of the mask.
U.S. Pat. No. 3,887,828 suggests applying to the metallic perforated mask a porous manganese dioxide layer and a thin layer of metallic aluminum on top thereof. The aluminum layer has contact with the shadow mask at the aperture edges only. It is electrically conducting and has an electron-absorbing property. Top of the aluminum layer is another layer of graphite, nickel oxide, or nickel iron.
The porosity of the manganese oxide layer is said to originate substantially from the individually arranged particles, the layer being sandwiched by the mask and thin aluminum layer. Due to the layer structure, heat generated by electron impact is intended to be kept away from the metallic perforated mask and emitted in the opposite direction.
This solution has various drawbacks. It has shown that keeping the generated heat away from the perforated mask is not feasible since the major part of the heat is not generated within the aluminum layer and the overlying graphite layer, but in the perforated mask. The electron-reflecting, electron-absorbing, and heat-emitting properties of the aluminum layer are too low. The heat-insulating sandwich structure arranged on top of the perforated mask now results in the opposite effect: The heat can be emitted only with difficulty.
DE 3,125,075 C2 describes an electron-reflecting layer directly coating the shadow mask. This layer contains heavy metals, particularly in the form of their carbides, sulfides or oxides. On electron impact, up to 30% of the electrons can be reflected, which means that the shadow mask is less heated. However, the major part of the electron beam still reaches the shadow mask, giving rise to undesirable heat generation therein and thus, general and local doming phenomena of the shadow mask.
U.S. Pat. No. 4,671,776 suggests coating the shadow mask with borate glass. The glass powder is sprayed onto the mask and is subsequently melted. The glass layer adheres very tightly to the backing. In operating conditions, the doming effect is diminished due to some heat-insulation but the major effect is from tensile forces within the mask resulting from the different expansion coefficients of the layer and the metal of the shadow mask. With such a coating, electron-reflecting effects can hardly be observed, so that a major part of the energy of the impacting electron beam still is transferred to the mask, giving rise to disadvantageous doming behavior.
Moreover, rigid fixation of the mask with a stable glass layer does not meet the higher requirements with respect to color picture quality in the multimedia age.
Another means of significantly limiting the undesirable doming phenomena is the use of high quality metal alloys, such as Invar, for the shadow mask, because this alloy has a particularly favorable thermal expansion coefficient. However, this material is highly expensive with respect to costs.
Moreover, since the cost percentage of the shadow mask with respect to the total cost of a color picture tube is already relatively high, the use of special metal alloys would result in a further increase of costs.